A wave of street fighting returned to Gaza yesterday as the Palestinian interior minister resigned in frustration at the worsening dispute between the rival Hamas and Fatah factions.

Hani al-Qawasmi, an independent bureaucrat who had been given one of the toughest jobs in the new coalition government, quit after failing to impose much-needed security reforms. "I told all parties I cannot accept being a minister without authority," he told a news conference.

At least four Palestinians were killed in fighting in Gaza yesterday. So far nine have died and more than 50 have been injured since the weekend in a round of tit-for-tat shootings and abductions. Egyptian mediators had tried to implement the latest of several ceasefires, but it fell apart almost as soon as it had been agreed.

The shootings were the most serious confrontation between Hamas and Fatah gunmen since the two factions came to an agreement three months ago in Mecca. Their deal had come after more than 100 had been killed in the worst infighting in years, raising fears that the conflict would escalate into outright civil war.

The latest shootings followed a decision last week by Mahmoud Abbas, the Palestinian president and Fatah leader, to deploy 3,000 of his security forces in Gaza as policemen. On Sunday Baha Abu Jarad, a senior figure in the al-Aqsa Martyrs Brigade, an armed group linked to the Fatah movement, was shot dead with his bodyguard. His killing was blamed on Hamas gunmen and led to a series of shootings between men from Fatah and Hamas. The two groups have sharp ideological differences, but are also locked in a deadly struggle for power.

Mr Qawasmi had tried to resign two weeks ago, in protest at the continued rivalry between the two factions, but his resignation was refused. This time Ismail Haniyeh, the Palestinian prime minister and a Hamas leader, had little choice but to accept.

Hamas won elections in January last year, but with its hardline Islamist agenda has been boycotted by Israel and western countries. Fatah, the more moderate movement, which was pushed out of power for the first time in recent history, wants to reassert its influence through its several security forces.

The United States has promised $59m (£30m) to reform and improve the security forces loyal to Mr Abbas and strengthen his position against his Hamas rivals. Hamas has reportedly responded by trying to enlarge its armed Executive Force, bringing in more weapons and providing more training.

Yesterday's resignation underlines the fragility of the coalition government formed after the Mecca agreement. Not only has the new government failed to contain the lawlessness that has engulfed Gaza, it has also failed to lift the international boycott imposed last year. In Brussels yesterday, the EU external relations commissioner, Benita Ferrero-Waldner, said there had been "quite encouraging signals" from the Palestinian government, but the EU was still not ready to allow direct funding.

Even at the time Mr Qawasmi was appointed, some Palestinian political leaders said they doubted he had the influence to rein in rival groups. Sufian Abu Zaida, a former minister and senior Fatah leader from northern Gaza, said at the time it was clear Mr Qawasmi had no experience handling security issues and was chosen as a weak compromise candidate.